N-Methylmorpholine oxide is a proven solvent for cellulose for the production of cellulose fibers, e.g. U.S. Pat. No. 3,447,939. It is further used as an oxidizing agent in the dihydroxylation of olefins, See, Sharpless et al., J. Am. Chem. Soc. 98 1976!, 1986 and finds utility in intermediate production for pharmaceuticals. The preferred commercial form is an aqueous solution from 50 to 60% in strength.
N-Methylmorpholine oxide, or tertiary amine oxides, in general, are prepared by oxidizing the corresponding amines with hydrogen peroxide, as described in EP 553 552, U.S. Pat. No. 4,247,480, EP 426 084, FR 26 32 638, EP 401 503, U.S. Pat. Nos. 5,055,233, 5,075,501, 5,130,488, EP 307 184, U.S. Pat. No. 4,994,614, EP 320 694, EP 356 918, DE 36 18 352, U.S. Pat. No. 4,748,241, EP 498 346, EP 424 965, U.S. Pat. No. 4,970,341, incorporated by reference herein.
The general reaction scheme for the preparation of NMMO is: ##STR1##
Nitrosamines and colored species are reaction byproducts which negatively impact product quality, even in low concentrations. Nitrosamines are undesirable even in trace amounts because of their carcinogenicity and much effort has been devoted to the suppression of nitrosamine formation, e.g. EP 553 552, U.S. Pat. Nos. 4,247,480, 4,994,614, EP 356 918. However, little attention has been paid to the formation of the colored species when the N-methylmorpholine is oxidized with hydrogen peroxide in the presence of an inert gas or a carbon dioxide atmosphere.
According to U.S. Pat. No. 4,748,241, the color of the resulting NMMO solution can be reduced from "deeply yellow-brown" to "slightly tinged yellow" by purifying the N-methylmorpholine by an azeotropic distillation with water. The reaction of the NMM-water azeotrope (74.5% NMM, 25.5% water) with aqueous 30-65% hydrogen peroxide is said to lead only to slightly yellowish NMMO solutions.
The reaction of undiluted NMM-water azeotrope with 30-65% aqueous hydrogen peroxide is illustrated in Examples 1-8. The NMMO solutions obtained from Examples 1-8 all had APHA color numbers above 350 units and are not suitable for all applications unless further purification steps are undertaken. The APHA color number is a standard method based on a visual comparison of the sample with solutions with known concentrations of cobalt chloroplatinate. The unit of color is that produced by 1 mg platinum/L in the form of the chloroplatinate ion.
Applicants' invention represents an improvement over the art because the reaction of hydrogen peroxide with NMM azeotrope (about 26% water) results in highly colored NMMO solutions as illustrated in Examples 1-8. Surprisingly, Applicants have reduced the color of the NMMO solution by increasing the water content of the NMM solution to at least 35% prior to the addition of hydrogen peroxide. Examples 9-23 are illustrative of the Applicants' invention and its superiority over the art.